Actually there are so many ways to say you suck at something.
Tanit's are good: I don't know beans about that. Very informal, but not at all vulgur. I know squat about that. (Also informal, also not vulgar.) (You may remember my old sig line: I don't know squat about Type II conditionals...) Interestingly, I don't know squat and I know squat both mean you know nothing.
If I heard someone say "I'm a real zero when it comes to that" I would think that if he were grading himself on a scale from 1 to 10, he'd not even give himself a 1, and your meaning would come through.
Now, of course, I hear Butterfly McQueen and her famous line "I don't know nuthin' 'bout birthin' no babies!"
My favorite is "I'm totally clueless when it comes to ..." (cars, PCs, cricket).
And of course, plan English: I have no skills in that area at all!